Radio frequency identification (RFID) systems typically use one or more RFID reader antennae to send radio frequency (RF) signals to items tagged with RFID tags. The use of such RFID tags to identify an item or person is well known in the art. In response to the RF signals from a RFID reader antenna, the RFID tags, when excited, produce a disturbance in the magnetic field (or electric field) that is detected by the RFID reader antenna. Typically, such tags are passive tags that are excited or resonate in response to the RF signal from a RFID reader antenna when the tags are within the detection range of the RFID reader antenna.
The detection range of the RFID systems is typically limited by signal strength to short ranges (or detection zones), for example, frequently less than about one foot for 13.56 MHz systems. Therefore, portable reader units may be moved past a group of tagged items in order to detect all the tagged items, particularly where the tagged items are stored in a space significantly greater than the detection range of a stationary or fixed single RFID reader antenna. Alternately, a large RFID reader antenna with sufficient power and range to detect a larger number of tagged items may be used. However, such an antenna may be unwieldy and may increase the range of the radiated power beyond allowable limits. Furthermore, these RFID reader antennae are often located in stores or other locations where space is at a premium and it is expensive and inconvenient to use such large RFID reader antennae. In another possible solution, multiple small antennae may be used, but such a configuration may be awkward to set up when space is at a premium and when wiring is preferred to be hidden.
Current RFID reader antennae are designed so that a sufficient read range may be maintained between the antenna and associated tags, without running afoul of FCC limitations on radiated emissions.
Resonant loop RFID reader antenna systems are currently utilized in RFID applications, where numerous RFID reader antennae are connected to a single reader. Each RFID reader antenna may have its own tuning circuit that is used to match to the system's characteristic impedance. A number of moderately sized antennae may be used, where the coverage of a given antenna may extend somewhat beyond that antenna's bounds, and may even overlap the coverage of one or more adjacent antennae, in order not to miss tags near the edges of an antenna or along boundaries between two antennae. If more than one antenna can read a given RFID tag, duplicate reads may arise for that tag. This may result in an uncertainty as to the location of the tagged item. It may also increase the data traffic due to duplicated or repeated transmissions of data for that tag.